1 gbps wireless link in 5 ghz radio frequency band
DESCRIPTION
WLAN link using 5GHz unlicensed spectrum providing 1Gbps throughput.TRANSCRIPT
RF Circuits and Systems (EEE 6374 Spring 2013) Project Report; UFID#06078296 1
Abstract—Wireless Local Area Network (WLAN) are an
extension of Local Area Network (LAN) which are typically a
short ranged data communication network between portable
devices and an Access Point (AP). The end users or nodes,
communicate to this AP using a Network Adapter analogous to
Ethernet Connection. The mobility within transmission range
gives added advantage to being connected. Fast growing
requirement of Gigabit data rates, call for sophisticated network
architecture reform from PHY (MIMO-OFDM) to Transport
layer (Wireless TCPs) and high level modulation ensuring more
data rate, but it comes inherently with higher Bit Error Rate
(BER) and poorer EVM (Error Vector Magnitude-Signal
Constellation) at the transmitter. In this project, an RF wireless
Link has been designed which ensures to provide peak data rate
of 1 Gbps at 5 GHz band. High attenuation is expected since
frequency band is high; therefore the distance of transmission is
also a major concern. RF system design for transmitter and
receiver is presented, to keep EVM within 2% and BER within
10-5 which are the prescribed limits as mentioned in subsequent
sections.
Index Terms— Path Loss, Power Amplifiers, QAM, Signal
Constellation, Spectrum.
I. INTRODUCTION
IRELESS LAN RF module is the most important aspect
in a Digital Communication System, since it has a direct
impact on the quality of receiver signal. This part is
responsible to move the baseband data onto Radio Frequency,
in which it is transmitted. Since we operate in a wireless
medium, the received signal is typically attenuated and
distorted likewise due to Multi-paths caused by reflection and
refraction in the environment. High Level modulation schemes
such as QPSK, 16/64/256 QAM can be used which pumps
more bits per symbol, but these increase the Bit Error Rate
(BER) for a given SNR (we denote SNR as in digital
domain). The signal constellation gets crowded as we increase
the modulation level, thus making it vulnerable to decision
errors at the receiver decision device. If the transmitted power
is increased so as to increase the probability of the received
signal to be more than a threshold SNR, we tend to increase
the noise at the transmitted signal too along-with putting more
out of band emissions which might cause interference to the
adjacent channels. There are several such considerations
which are described in the following sections.
At the transmitted, we intend not to saturate the PA (Power
Amplifier) and IF Amplifier (Intermediate Frequency). This is
the reason for the OIP3 (3rd
Order Output Intermodulation
Product) to be kept on higher side, especially for the PA, since
chances of saturating PA is highest, as the available input
power to the PA faces high Gain (at each stage) till it reaches
PA. Major concern in designing the entire Wireless Link is the
Linearity and Noise Figure. So, the link budget is designed
using Syscalc 6.0, where we only calculate the receiver Noise
Figure of receiver circuit and check for linearity and path loss
through receiver, at the transmitter. An important thing to note
is, we have to consider the transmitter and receiver antenna to
be Isotropic (0 dB Gain) as per given problem statement.
For designing an RF module, we need to consider one
frequency band which can provide 1Gbps data rate. The center
frequency should be in the 5GHz band. Therefore 256 QAM
has been considered as a modulation scheme at the transmitter
and a channel bandwidth of 160 MHz as specified in IEEE
802.11ac standard has been used. The RF transmitter and
receivers has been designed using Agilent ADS 2012.08 using
heterodyne architecture because of several reasons which will
be explained throughout the flow of this work.
Section II consist a basic overview of the channel allocation
and band specification as mentioned in IEEE 802.11ac.
Section III presents the transmitter and receiver architecture.
This section explains all the modules being used and their
specification has been justified with the application point of
view. In section IV, Link budget analysis has been shown.
This has been implemented using Syscalc 6.0 and the values
has been used to manually calculate receiver sensitivity, since
it is observed, this software does not provide appropriate
Sensitivity. Section V concludes the work and followed by
acknowledgement in section VI.
II. CHANNEL ALLOCATION AND SPECTRUM SPECIFICATIONS –
IEEE 802.11ac
IEEE 802.11ac has been standardized in the IEEE 802.11
suite, which ensures to give a peak data rate of 3.4 Gbps
(single link). The FCC transmit spectrum mask has been
shown in fig.. . Essentially it uses MIMO spatial streams to
boost up the data rate. The channel allocation is shown in
fig.1. FCC UNII spectrum bands are shown in the following
figure. 802.11ac takes up the 160MHz band with the given
center frequency.
RF Architecture for 1 Gbps Wireless Link
WLAN.
Ankan Roybardhan, Department of Electrical and Computer Engineering,
University of Florida, Gainesville, FL(USA).
W
RF Circuits and Systems (EEE 6374 Spring 2013) Project Report; UFID#06078296 2
.
Fig.1. IEEE802.11 spectrum allocation from FCC-UNII
For our work, would consider the 160MHz band, and one
band would include one channel (Not considerable out-of-
band emission). The consideration is quite logical in the sense,
since we are not implementing sophisticated MIMO
techniques, for getting 1Gbps data rate we need to consider
huge bandwidth (160MHz X 8 bits per symbol for 256
QAM=1.2 Gbps).
The standards [7][8] shows the maximum transmitted power
should not exceed +30 dBm, which is the case we get in our
simulation. The transit mask should follow the following-
Fig.2. Transit Spectrum Mask
Center frequency is considered to be 5.25GHz (5170+80
MHz). Channel BW is 160 MHz and so the band 160 MHz as
it is a wide band channel and we typically follow 802.11ac
standard. In addition to it, since the channel allocation is not
contiguous as in fig.1, we would consider one single channel
as a whole because a part of the 2nd
channel has weather
forecast communications.
III. RF SYSTEM ARCHITECTURE
Heterodyne architecture has been considered for this design
process because the circuit simplicity. We are essentially
operating at 5GHz band, so chances of LO leakage, and DC
offset may come up which may produce a zero frequency
component and ultimately saturate the high speed ADC, if
homodyne system had been implemented. It is better to go for
atleast one stage down-conversion to the baseband before we
demodulate or put any kind of filtering (higher operating
frequency will give high Q filter which is costly).
Following are architectures explained for the Wireless
transmission system-
A) Transmitter RF architecture
Modulation-256 QAM : 8 bits per symbol.
The Intermediate Frequency (IF) is 290 MHz because the IF
should be more than the receiver band (160 MHz) and the
practical blocks/modules in transmitter are available in market
for this IF frequency. Symbol rate is 125 MHz (1 Gbps/8 bits
per symbol). Inside the SUC_AMUFA has
been shown below –
Fig.4. IF-RF Composite Block with Channel Select Filter
Following are the list of the components used in the
architecture-
Block/Components Gain
(dB)
Noise Figure
(dB)
OIP3
(dBm) F_center
Filter
Passband
IF Amplifier 21.9 3.7 34.6
Mixer 0.8ǂ 13.2* 24
LO =4.9GHz;
and IF =
290 MHz
Bandpass
Filter1_chebyshev -2.5** 2.5 5.25 GHz 225 MHz
Power Amplifier
(PA) 32 3.01 42.636
Duplexer
Filter_Elliptic*** -2.5** 2.5 5.25 GHz 225 MHz
*NFSSB=NFDSB-3dB=16.2-3 dB.
**Gain= - Insertion Loss.
*** Elliptical filter gives the optimal response amongst all IIR filter classes. ǂ Mixer has conversion Gain only. ConvGain.
The band selected is 225 MHz. This is because of the fact
that it is wide band system and it operates on higher frequency
band 5GHz, which is not crowded much with ISM equipment
or other part 15 equipment, the filter response can be relaxed.
This relaxation is used to meet/adjusted to meet the EVM
requirement of maximum 2%.
B) Receiver RF Architecture.
Fig.5. Receiver RF architecture
The main idea here is, we would emulate the received signal
constellation as shown in the above figure. by a 256 QAM
system with a given RF Power calculated equal to sensitivity.
f1=79 MHz
f2=81 MHz
f3=160 MHz
f4=240 MHz
Fig.3. Transmitter RF architecture
RF Circuits and Systems (EEE 6374 Spring 2013) Project Report; UFID#06078296 3
Though the constellation received will be poor at the receiver,
but this is only way we can consider having the received
signal at the receiver’s input. The received power which is set
to -52.32 dBm is calculated by the sensitivity equation as
explained in section IV. Here we use, Lower Side Band (LSB)
receiver circuit, so the LO_frequency is set to 5.54GHz such
that different between LO and RF should be IF=290 MHz. The
block SDC_AMFA is enlarged as following –
Fig.6. Receiver RF architecture
Following are the list of the components used in the
architecture-
Block/Components Gain
(dB)
Noise
Figure (dB)
OIP3
(dBm) F_center
Filter
Passband
Duplexer
Filter_SAW 21.9 3.7 34.6
Low Noise
Amplifier 24.43 1.86 23
Mixer 0.8 16.3* 24
LO
=5.54GHz; and IF =
290 MHz
Bandpass Filter1_chebyshev
-1.3 1.3 290 MHz 225 MHz
IF Amplifier 21.9 3.7 34.6
*Here we do Single Side band. So NF will be 3 dB worse
IV. LINK BUDGET ANALYSIS
Fig. 7a. SysCalc Linearity Check for Transmitter
1) BER is given as 10-5
; Eb/N0 =22.4dB
( )
**T=290K; KT=-174 dBm/Hz.
2) Maximum transmission range
By frii’s free space equation - ( ) ( ) ( ) ( ) ( )
Since, Pt =19dBm from the simulation result as below of the transmitter circuit
and the P input for this transmitter is from syscalc Pin-
Fig.7b. Power Amplifier Pin v/s Pout 1st and 3rd order IMP
3) PA Modeling
Fig. 8. Power Amplifier Pin v/s Pout 1st and 3rd order IMP
The point has been shown for the 1dB compression for the
1st and 3
rd order Intermodulation Product.
It is seen that Interpolated Output Power (P1dB,out dBm), gain
(dBm) and Gain Compression (dB) are 30.976, 31.014 and
0.962 respectively. Which means by P1dB,out=P1dB,in+G-0.962,
P1dB,in =0.962 dBm where in actual, the input power to PA is
(19.87 dBm+2.5(due to filter)-32(gain of PA)=-9.63 dBm), so
the PA will not saturate with this input Power to the PA.
4) Signal Constellation – Transmitter
Fig.9. Transmitter EVM
RF Circuits and Systems (EEE 6374 Spring 2013) Project Report; UFID#06078296 4
5) Signal Constellation-Receiver
Fig.10. Receiver Signal Constellation
The constellation is degraded due to free space propagation
losses. This is intuitive, since for such a higher QAM system,
the constellation should have degraded this much. Efficient
Coding schemes and MIMO techniques are applied at PHY to
mitigate this effect.
6) Spectrum at receiver
Fig.11. Receiver Signal Spectrum
The receiver spectrum is shown as above. The out of band
emissions are pretty low around the center frequency.
V. CONCLUSION
The prime concern in design of this RF WLAN link was to
observe the Non Linearity at different points. The output of
the transmitter is 19.87 dBm which will push the range of
transmission to a maximum of around 21 mtrs. We had
detailed modeling of the Power Amplifier since its 3rd
order
Non Linearity is important as the available input power is high
enough to drive it saturation. At the receiver end, the LNA
should have the low Noise Figure (being the 1st stage) i.e. 1.5
dB which is good enough. The filter BW is not so strict for the
fact that spectrum is not much crowded as traditional 2.45
GHz band. The EVM is perfectly satisfied with 1.8% and the
sensitivity is around -53dBm. The part 15 FCC rules has been
satisfied by that fact that out of band emission is lower than
20dB @ 81MHz from center frequency. The constellation at
the receiver has degraded because of considerable SNR
degradation when passing through the medium. The Insertion
Loss and Stop band attenuation are the key factors for filter
selection. That has been satisfied by the filters found in
market.
VI. ACKNOWLEDGEMENT
This project is a partial requirement towards completion of
the course, EEE6374 RF Circuits and Systems. We thank Dr.
Jenshan Lin and the Mr. Taesong Hwang (TA) for guiding us
with every little details about the work and providing valuable
inputs throughout the time of working on this project.
VII. REFERENCES
[1] Shairi, N.A.; Rahman, T.A.; Abd Aziz, M.Z.A., "RF transmitter system
design for Wireless Local Area Network bridge at 5725 to 5825 MHz," Computer and Communication Engineering, 2008. ICCCE 2008. International
Conference on , vol., no., pp.109,112, 13-15 May 2008.
[2] Zhongming Shi; Rofougaran, R., "A single-chip and multi-mode 2.4/5GHz RF transceiver for IEEE 802.11 wireless LAN," Microwave and Millimeter
Wave Technology, 2002. Proceedings. ICMMT 2002. 2002 3rd International
Conference on , vol., no., pp.229,232, 17-19 Aug. 2002. [3] Shairi, N.A.; Ibrahim, I.M.; Rahman, T.A., "Third order intermodulation
distortion effect on the constellation error in RF transmitter of IEEE 802.11a
WLAN system," Industrial Electronics and Applications (ISIEA), 2011 IEEE Symposium on , vol., no., pp.223,226, 25-28 Sept. 2011.
[4] Office of Engineering and Technology - Federal Communications
Commission, UNDERSTANDING THE FCC REGULATIONS FOR LOW-POWER, NON-LICENSED TRANSMITTERS, USA.
[5] Pozer, D.M., Microwave and RF Design of Wireless Systems, John Willey
& Sons, Inc. NY. [6] Razavi, B., RF Microelectronics, Prentice Hall Communications and
Emerging Technologies Series, NJ.
[7]http://www.mhprofessional.com/downloads/products/0071701524/0071701524_chap02.pdf-WLAN standard
[8]http://www.litepoint.com/whitepaper/80211ac_Whitepaper.pdf-IEEE
802.11ac standard white paper. [9] Jenshan Lin, Lecture notes, Spring 2013.
VIII. COMPONENT LISTING
Component Make Power
Consumption
Parameters
Pricing
Tx_Rx_IF
Amplifier
MiniCircuits-
CMA 63+
0.725 W $ 4.95
Tx_Mixer Analog Devices ADL5801
0.161W $ 6.18
Tx_Chebyshev MiniCircuits-
VBF 7200+
2.01W $34.95
Tx_Power Amplifier
Skyworks – SE5003L-1*
0.3W $11.00
Tx_Duplex_filt
er.
MiniCircuits-
VBF 7200+
2.01W $34.95
Rx_Duplex_filter.
MiniCircuits-VBF 7200+
2.01W $34.95
Low Noise
Amplifier (LNA)
Skyworks-
SKY65404_31_201512I*
0.302W
Tx_Mixer Analog Devices
ADL5801
0.161W $6.18
Rx_BPF_Chebyshev Filter
MiniCircuits-RBP-253+
$15.95
*The component has been specially designed for 802.11ac radios.